Method for preparing lithium iron phosphate nanopowder

ABSTRACT

The present invention relates to a method for preparing a lithium iron phosphate nanopowder, including the steps of (a) preparing a mixture solution by adding a lithium precursor, an iron precursor and a phosphorus precursor in a reaction solvent, and (b) putting the mixture solution into a reactor and heating to prepare the lithium iron phosphate nanopowder under pressure conditions of 1 to 10 bar, and a lithium iron phosphate nanopowder prepared by the method. When compared to a common hydrothermal synthesis method, a supercritical hydrothermal synthesis method and a glycothermal synthesis method, a reaction may be performed under a relatively lower pressure. Thus, a high temperature/high pressure reactor is not necessary and process safety and economic feasibility may be secured. In addition, a lithium iron phosphate nanopowder having uniform particle size and effectively controlled particle size distribution may be easily prepared.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for preparing a lithium ironphosphate nanopowder.

2. Description of the Related Art

As technical development and consumption of mobile devices increase, thedemand for secondary batteries as energy sources is suddenly increasing.Among such secondary batteries, lithium secondary batteries having highenergy density and voltage, a long life cycle, and a low self-dischargerate are being commercialized and widely used.

Common lithium secondary batteries use lithium cobalt oxide (LiCoO₂) asthe main component of a cathode active material. However, since thecobalt oxide containing lithium is unstable and expensive, the massproduction of lithium secondary batteries including thereof isdifficult.

Recently, lithium iron phosphate (LiFePO₄) compound having a voltageagainst lithium of ˜3.5 V, a high volume density of 3.6 g/cm³, and atheoretical capacity of 170 mAh/g, as well as good stability at hightemperature, and being cheap when compared to the lithium cobalt oxide,is being viewed as a suitable cathode active material for a lithiumsecondary battery.

As methods for preparing the lithium iron phosphate compound, asolid-state reaction method or a liquid-state reaction method such as ahydrothermal synthesis method and a supercritical hydrothermal synthesisis known. Recently, a glycothermal synthesis method is using anon-aqueous solvent such as ethylene glycol or diethylene glycol as areaction solvent has been developed. According to the hydrothermalsynthesis method and the supercritical hydrothermal synthesis method,the preparation of the lithium iron phosphate nanopowder is performedunder high temperature and high pressure conditions, giving rise tosafety concerns. In addition, according to the glycothermal synthesismethod, the control of the particle size and the particle sizedistribution of the lithium iron phosphate nanopowder may be difficult.

SUMMARY OF THE INVENTION

An aspect of the present invention provides a method for preparing alithium iron phosphate nanopowder using a novel reaction solvent toresolve the above-described limitations, by which a reaction may beperformed under relatively low pressure conditions when compared to acommon hydrothermal synthesis method, a supercritical hydrothermalsynthesis method, or a glycothermal synthesis method. Thus, a hightemperature/high pressure reactor may not be used, so that processsafety and economic feasibility may be secured, while uniform particlesize may be obtained, and a particle size distribution may becontrolled.

According to an aspect of the present invention, there is provided amethod for preparing a lithium iron phosphate nanopowder including (a)preparing a mixture solution by adding a lithium precursor, an ironprecursor and a phosphorus precursor in a reaction solvent, and (b)putting the reaction mixture into a reactor and heating to form alithium iron phosphate nanopowder under pressure conditions of 1 to 10bar.

According to another aspect of the present invention, there is provideda lithium iron phosphate nanopowder prepared by the method, and acathode active material including the same.

According to still another aspect of the present invention, there areprovided a cathode including the cathode active material and a lithiumsecondary battery including the cathode.

According to the method for preparing a lithium iron phosphatenanopowder of the present invention, a reaction may be performed underrelatively lower pressure conditions when compared to a commonhydrothermal synthesis method, a supercritical hydrothermal synthesismethod and a glycothermal synthesis method, such that a hightemperature/high pressure reactor may not be used and so, process safetyand economic feasibility may be secured, while uniform particle size maybe obtained and particle size distribution may be controlled.

A lithium secondary battery including the lithium iron phosphatenanopowder thus prepared as a cathode active material has good capacityand stability.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction (XRD) pattern of a lithium iron phosphatenanopowder prepared according to an embodiment of the present invention(Examples 1a and 1b);

FIG. 2 illustrates photographic images of a scanning electron microscope(SEM) of lithium iron phosphate nanopowders prepared according toembodiments of the present invention (Examples 1a and 1b);

FIG. 3 illustrates particle size distribution of a lithium ironphosphate nanopowder prepared according to an embodiment of the presentinvention (Example 1b);

FIG. 4 illustrates photographic images of a transmission electronmicroscope (TEM) of a carbon coated lithium iron phosphate nanopowderprepared according to an embodiment of the present invention (Example1c);

FIG. 5 is an X-ray diffraction (XRD) pattern of a lithium iron phosphatenanopowder prepared according to an embodiment of the present invention(Example 2);

FIG. 6 is a photographic image of a scanning electron microscope (SEM)of a lithium iron phosphate nanopowder prepared according to anembodiment of the present invention (Example 2);

FIG. 7 is an X-ray diffraction (XRD) pattern of a lithium iron phosphatenanopowder prepared according to an embodiment of the present invention(Example 3);

FIG. 8 is a photographic image of a scanning electron microscope (SEM)of a lithium iron phosphate nanopowder prepared according to anembodiment of the present invention (Example 3);

FIG. 9 is an X-ray diffraction (XRD) pattern of a lithium iron phosphatenanopowder prepared according to a comparative embodiment (ComparativeExample 1); and

FIG. 10 is a photographic image of a scanning electron microscope (SEM)of a lithium iron phosphate nanopowder prepared according to acomparative embodiment (Comparative Example 1).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Exemplary embodiments of the present invention will now be described indetail.

In the present invention, a method for preparing a lithium ironphosphate nanopowder using a novel reaction solvent under relatively lowpressure conditions, is provided to resolve the safety issue and highcost brought about in a hydrothermal synthesis method, a supercriticalhydrothermal synthesis method, or a glycothermal synthesis method, inwhich a reaction is performed under high temperature/high pressureconditions, so an high temperature/high pressure reactor (for example,autoclave) is used. According to the present invention, process safetyand the economic feasibility may be largely improved when compared tothe common hydrothermal synthesis method, the supercritical hydrothermalsynthesis method, or the glycothermal synthesis method, and a lithiumiron phosphate nanopowder having uniform particle size may be obtained.

In an embodiment for attaining an aspect of the invention, a method forpreparing a lithium iron phosphate nanopowder including (a) preparing amixture solution by adding a lithium precursor, an iron precursor and aphosphorus precursor in a reaction solvent, and (b) putting the reactionmixture into a reactor and heating to form a lithium iron phosphatenanopowder under pressure conditions of 1 to 10 bar, is provided.

In another embodiment of the present invention, the method may furtherinclude (c) heat treating the lithium iron phosphate nanopowder thusprepared to form a coating layer on a portion or the whole of thesurface of the individual particle of the nanopowder.

First, a lithium precursor, an iron precursor, and a phosphorusprecursor are prepared and added in a reaction solvent to prepare ahomogeneous mixture solution (Step (a)).

The lithium precursor thus added may be at least one selected from thegroup consisting of lithium acetate dihydrate (CH₃COOLi.2H₂O), lithiumhydroxide monohydrate (LiOH.H₂O), lithium hydroxide (LiOH), lithiumcarbonate (Li₂CO₃), lithium phosphate (Li₃PO₄), lithium phosphatedodecahydrate (Li₃PO₄.12H₂O) and lithium oxalate (Li₂C₂O₄), and amixture thereof.

The iron precursor added may be at least one selected from the groupconsisting of iron citrate (FeC₆H₅O₇), iron citrate hydrate(FeC₆H₅O₇.nH₂O), ferrous sulfate heptahydrate (FeSO₄.7H₂O), iron(II)oxalate dihydrate (FeC₂O₄.2H₂O), iron acetyl acetonate (Fe(C₅H₇O₂)₃),iron phosphate dihydrate (FePO₄.2H₂O) and ferric hydroxide (FeO(OH)),and a mixture thereof.

The phosphorus precursor added may be at least one selected from thegroup consisting of tri-ammonium phosphate trihydrate ((NH₄)₃PO₄.3H₂O),ammonium phosphate ((NH₄)₂HPO₄), ammonium dihydrogen phosphate(NH₄H₂PO₄) and phosphoric acid (H₃PO₄), and a mixture thereof.

Meanwhile, the mixing ratio of the lithium precursor, the iron precursorand the phosphorus precursor during the forming of the mixture solutionis not specifically limited, and may be, for example, 0.1-10:1:0.1-10 bythe molar ratio.

In addition, the iron precursor may be added from about 0.005 to about 1parts by weight based on 1 part by weight of the reaction solvent, andthe lithium precursor and the phosphorus precursor may be added bycorresponding molar ratios considering the amount of the iron precursor.

A stirring step may be further conducted during preparing the mixturesolution so that the lithium precursor, the iron precursor and thephosphorus precursor may be homogeneously dispersed in the reactionsolvent.

Meanwhile, the reaction solvent may be a butandiol solvent which is anon-aqueous solvent and has a relatively high boiling point.

In an embodiment of the present invention, the reaction solvent mayinclude at least one selected from the group consisting of1,2-butandiol, 1,3-butandiol, 1,4-butandiol, 2,3-butandiol and an isomerthereof.

The boiling point of the 1,2-butandiol is about 195° C., the boilingpoint of the 1,3-butandiol is about 204° C., the boiling point of the1,4-butandiol is about 235° C., and the boiling point of the2,3-butandiol is about 177° C.

The lithium iron phosphate nanopowder was verified to be synthesized attemperature conditions of at least about 150° C.

That is, when the above-described reaction solvents are used forpreparing the lithium iron phosphate nanopowder, the reaction may beperformed at a temperature less than or equal to the boiling point ofthe reaction solvent, and the vaporization degree of the reactionsolvent may decrease. Thus, the pressure increase due to thevaporization of solvents during the performing of the reaction may besignificantly suppressed when compared to a common hydrothermalsynthesis method. Therefore, safety throughout the process may beimproved.

In addition, since the above described solvents are non-aqueous, theproblem of iron oxidation may be solved without using a separatereducing agent.

Then, the mixture solution was put into a reactor and heated to preparethe lithium iron phosphate nanopowder under the pressure conditions of 1to 10 bar (Step (b)).

The reactor may be a generally used reactor and may be an open typereactor or a closed-type reactor connected to a reflux equipment, inwhich the pressure is not high, but is not limited thereto.

That is, the pressure conditions in Step (b) do not specifically requirea pressure-resistant vessel that withstands a particularly hightemperature and a high pressure. Thus, unlike the common hydrothermalsynthesis method (at least about 100 bar), the supercriticalhydrothermal synthesis method (at least about 220 bar) or theglycothermal synthesis method (from about 10 to about 100 bar), in whichthe use of a pressure-resistant vessel is necessary for preparing thelithium iron phosphate nanopowder, the method of the present inventiondoes not require a high-pressure-resistant reactor, thereby improvingprocess safety and economic feasibility.

Meanwhile, the above Step (b) may be performed at the temperature rangeof at least 150° C. which is the minimum temperature for synthesizingthe lithium iron phosphate nanopowder, and at a temperature range lessthan or equal to the boiling point of the reaction solvent used.

For example, when the reaction solvent used in the present invention is1,2-butandiol, the above Step (b) may be performed at 150 to 195° C.,and when the reaction solvent is 1,3-butandiol, the above Step (b) maybe performed at 150 to 204° C. In addition, when the reaction solventused in the present invention is 1,4-butandiol, the above Step (b) maybe performed at 150 to 235° C., and when the reaction solvent is2,3-butandiol, the above Step (b) may be performed at 150 to 177° C.

That is, the reaction is performed at a temperature between the minimumtemperature for synthesizing the lithium iron phosphate nanopowder and atemperature less than or equal to the boiling point of the reactionsolvent, and the vaporization of the reaction solvent is slowed down.Thus, the pressure increase of the reactor due to the vaporization ofthe solvent maybe suppressed when compared to a common hydrothermalsynthesis method, etc.

Meanwhile, the pressure in the reactor during the performing of theabove Step (b) is in the range of 1 to 10 bar. The pressure isrelatively lower when compared to that of the common hydrothermalsynthesis method (at least about 100 bar), of the supercriticalhydrothermal synthesis method (at least about 220 bar), or of theglycothermal synthesis method (from about 10 to about 100 bar), whichprovides even better effect when considering process safety and economicfeasibility.

The processing time of the above Step (b) may be changed according tothe reaction solvent used and the reaction temperature.

In an embodiment of the present invention, when 1,4-butandiol is used asthe reaction solvent, the above Step (b) may be conducted at atemperature range of 150 to 235° C. for 1 to 72 hours, and moreparticularly, may be conducted at a temperature range of 180 to 235° C.for 1 to 48 hours.

When Step (b) is finished, lithium iron phosphate nanopowder particlemay be synthesized, and a washing step and a drying step for recoveringthe synthesized lithium iron phosphate nanopowder particle in Step (b)may be conducted sequentially.

The washing method in the washing step is not specifically limited, andmay be conducted sequentially by using acetone and methanol.

The drying method in the drying step is not specifically limited, andmay be conducted at a temperature range of 20 to 160° C. for 2 to 40hours.

The lithium iron phosphate nanopowder synthesized through the processesmay be heat treated to forma coating layer on a portion or the whole ofthe surface of individual particle of the powder (Step (c)).

The above Step (c) may be performed through heat treating. The heattreating is not specifically limited and may be conducted by heating toa temperature range of 400 to 900° C., for example. Through the heattreating, a carbon coating layer or a coating layer composed of a glassylithium compound may be formed on a portion or the whole of the surfaceof the particle.

When the coating layer is the carbon coating layer, the precursor of thecoating layer may be the reaction solvent remaining on the surface ofthe particle after use. Particularly, the reaction solvent used mayremain on the surface of the particle after conducting the drying stepand may be carbonized during the heat treatment at a temperature rangeof 400 to 900° C., thereby forming the carbon coating layer on thesurface of the particle.

A separate organic compound may be used as the precursor of the carboncoating layer, and the addition step of the separate organic compoundfor forming the carbon coating layer on the surface of the lithium ironphosphate nanopowder particle is not specifically limited.

In an embodiment of the present invention, the organic compound may bemixed with the solvent together with the lithium precursor, the ironprecursor and the phosphorus precursor and react to form the carboncoating layer on the surface of the particle during the forming of thelithium iron phosphate particle.

In another embodiment, the lithium precursor, the iron precursor and thephosphorus precursor are mixed with the solvent and react to form thelithium iron phosphate particle, and then, the organic compound isadded, mixed and heat treated to form the carbon coating layer on thesurface of the particle.

In further another embodiment, the lithium precursor, the iron precursorand the phosphorus precursor are mixed with the solvent and react toform the lithium iron phosphate particle, and washing and dryingprocesses are performed. Thereafter, the organic compound is added,mixed and heat treated to form the carbon coating layer on the surfaceof the particle.

The organic compound is not specifically limited, and may be at leastone selected from the group consisting of glucose, sucrose, galactose,fructose, lactose, starch, mannose, ribose, aldohexose, ketohexose, anda combination thereof.

When the coating layer is the glassy lithium compound coating layer, thecoating layer is not specifically limited, and may be, for example, alithium phosphate-based amorphous coating layer. In this case, theprecursor material may be a lithium precursor and a phosphorusprecursor, and may be an additional lithium compound and phosphoruscompound.

The thickness of the carbon coating layer or the glassy lithium compoundcoating layer formed on the surface of the particle in this step is notspecifically limited, and may be, for example, less than or equal to 10nm.

Since the lithium iron phosphate powder has low electric conductivity,the electric conductivity of the lithium iron phosphate powder may beimproved by forming the carbon coating layer or the coating layerincluding the glassy lithium compound on a portion of or the wholesurface of the minute lithium iron phosphate powder particle thusprepared.

The lithium iron phosphate nanopowder particle prepared through theseries of the above-described steps may have an olivine structure.

The particle size and the particle size distribution of the particle maybe controlled by changing the lithium precursor, the iron precursor, orthe phosphorus precursor, or regulating processing variables such as areaction temperature and a reaction time, etc. For example, the size ofthe lithium iron phosphate particle may decrease when using lithiumacetate as the lithium precursor. In addition, the size of the lithiumiron phosphate may increase when the reaction temperature is elevated orthe reaction time is prolonged.

The particle size (Hereinafter will be referred to as particle diameter)of the lithium iron phosphate nanopowder prepared through theabove-described processes is not specifically limited, and may be, forexample, from 30 to 300 nm. The particle size distribution is notspecifically limited and may be, for example, less than or equal to 50%of the average value of the particle diameter.

Preparation of Lithium Secondary Battery

In the present invention, a cathode active material including thelithium iron phosphate nanopowder having the olivine crystal structuremay be provided. The cathode active material may further include aconductive agent, a binder and a filler other than the lithium ironphosphate powder selectively.

The conductive agent may include any material having conductivity andnot inducing a chemical change in a battery without specific limitation,and may include graphite such as natural graphite and syntheticgraphite; carbon blacks such as carbon black, acetylene black, ketchenblack, channel black, furnace black, lamp black, and thermal black;conductive fibers such as a carbon fiber and a metal fiber; metalpowders such as a carbon fluoride powder, an aluminum powder and anickel powder; conductive whiskers such as zinc oxide and potassiumtitanate; conductive metal oxides such as titanium oxide; and conductivematerials such as a polyphenylene derivative.

Generally, the conductive agent may be included by 1 to 30 wt % based onthe total amount of a mixture including the cathode active material.

The binder may be any component that assists the bonding of the activematerial and the conductive agent and the bonding with a currentcollector without specific limitation, and may include, for example,polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose(CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene,ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, a styrenebutadiene rubber, a fluorine rubber, various copolymers, etc.

Generally, the binder may be included by 1 to 30 wt % based on the totalamount of a mixture including the cathode active material.

The filler is a component suppressing the expansion of an electrode andmay be selectively used. The filler may be any fibrous material that maynot induce a chemical change in the battery, without specificlimitation, and may include, for example, an olefin-based polymer suchas polyethylene and polypropylene; and a fibrous material such as aglass fiber and a carbon fiber.

In addition, in an embodiment of the present invention, a cathode for alithium secondary battery obtained by coating the cathode activematerial on a current collector is provided.

The cathode for the lithium secondary battery may be fabricated by, forexample, dissolving the cathode active material in a solvent to preparea slurry, coating the slurry on the current collector, drying andpressing.

The current collector of the cathode may be any material havingconductivity and not inducing chemical change of a battery, withoutspecific limitation, and may include, for example, stainless steel;aluminum; nickel; titanium; fired carbon; or a surface treated materialof the aluminum or the stainless steel with carbon, nickel, titanium,silver, etc.

Generally, the current collector may have a thickness of 3 to 500 μm,and minute embossing may be formed on the surface of the currentcollector to increase the adhesiveness of the cathode active material.Various shapes such as a film, a sheet, a foil, a net, a porousmaterial, a foamed material, a non-woven fabric, etc. may be used as thecurrent collector.

In addition, in an embodiment of the present invention, a lithiumsecondary battery including a cathode including the cathode activematerial, an anode, a separator and anon-aqueous electrolyte containinga lithium salt may be provided.

The anode may be fabricated by, for example, coating an anode mixtureincluding an anode active material on an anode current collector, anddrying. In the anode mixture, the above-described components such as theconductive agent, the binder and the filler may be included as occasiondemands.

The anode current collector may be any material having high conductivityand not inducing the chemical change of a battery, without specificlimitation, and may include, for example, copper; stainless steel;aluminum; nickel; fired carbon; a surface treated material of copper orstainless steel with carbon, nickel, titanium, silver, etc.; and analloy of aluminum-cadmium.

Meanwhile, the current collector may have the thickness of 3 to 500 μm,and minute embossing may be formed on the surface of the currentcollector to increase the adhesiveness of the anode active material asin the cathode current collector. Various shapes such as a film, asheet, a foil, a net, a porous material, a foamed material, a non-wovenfabric, etc. may be used as the current collector.

The separator is disposed between the cathode and the anode, and aninsulating thin film having high ion transmittance and high mechanicalstrength may be used.

The pore diameter of the separator may be generally from 0.01 to 10 μm,and the thickness thereof may be generally from 5 to 300 μm.

The separator may include a chemical resistant and hydrophobicolefin-based polymer such as polypropylene; a sheet or a non-wovenfabric formed by using a glass fiber or polyethylene, etc.

When a solid electrolyte such as a polymer is used as the electrolyte,the solid electrolyte may also play the role of the separator.

The non-aqueous electrolyte containing the lithium salt includes theelectrolyte and the lithium salt, and the electrolyte may include anon-aqueous organic solvent or an organic solid electrolyte.

The non-aqueous organic solvent may include, for example, aproticorganic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate,ethylene carbonate, butylene carbonate, dimethyl carbonate, diethylcarbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydroxyfuran,2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide,dimethylformamide, dioxolane, acetonitrile, nitromethane, methylformate, methyl acetate, phosphoric acid triester, trimethoxymethane,dioxolane derivatives, sulfolane, methyl sulfolane,1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives,tetrahydrofuran derivatives, ether, methyl propionate, ethyl propionate,etc. The organic solid electrolyte may include, for example,polyethylene derivatives, polyethylene oxide derivatives, polypropyleneoxide derivatives, a phosphoric acid ester polymer, poly agitationlysine, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride,and a polymer containing an ionic dissociable group.

The lithium salt may include a material favorably soluble in thenon-aqueous electrolyte such as LiCl, LiBr, LiI, LiClO₄, LiBF₄,LiB₁₀C₁₀, LiPF₆, LiCF₃SO₃, LiCF₃CO₂, LiAsF₆, LiSbF₆, LiAlCl₄, CH₃SO₃Li,CF₃SO₃Li, (CF₃SO₂)₂NLi, chloroborane lithium, a low molecular weightaliphatic lithium carbonate, lithium 4-phenylborate, imide, etc.

In addition, the electrolyte may further include (for improving chargeand discharge properties, and flame retardance, etc.), for example,pyridine, triethyl phosphite, triethanolamine, a cyclic ether,ethylenediamine, n-glyme, hexaphosphoric acid amide, nitrobenzenederivatives, sulfur, a quinone imine dye, N-substituted oxazolidinone,N,N-substituted imidazolidine, an ethylene glycol dialkyl ether, anammonium salt, pyrrole, 2-methoxyethanol, trichloro aluminum, etc. Ahalogen-containing solvent such as carbon tetrachloride,trifluoroethylene, etc. may be further included to impartincombustibility, and a carbon dioxide gas may be further included toimprove preservation properties at a high temperature.

As described above, the method for preparing the lithium iron phosphatenanopowder of the present invention may be performed at relatively lowerpressure conditions when compared to a common hydrothermal synthesismethod, a supercritical hydrothermal synthesis method or a glycothermalsynthesis method by using a novel reaction solvent as a non-aqueoussolution. Thus, a high temperature/high pressure reactor is notnecessary, and process safety and economic feasibility may be secured, auniform particle size may be attained, and a lithium iron phosphatenanopowder having controlled particle size distribution may be easilyprepared.

In addition, a lithium secondary battery including the lithium ironphosphate nanopowder thus prepared as a cathode active material may havegood capacity and stability.

EXAMPLES

Exemplary embodiments of the invention will be described below in moredetail. The present invention may, however, be embodied in differentforms and should not be construed as limited to the embodiments setforth herein. Rather, these embodiments are provided so that thisdisclosure will be thorough and complete, and will fully convey thescope of the inventive concept to those skilled in the art.

Examples 1a and 1b

14.4 g of lithium hydroxide (LiOH), 147 g of ferric citrate (FeC₆H₅O₇)and 58.8 g of phosphoric acid (H₃PO₄) were added in 3 L of 1,4-butandioland sufficiently stirred to prepare a mixture solution.

The sufficiently stirred mixture solution was put into a 500 ml reactorand a reaction was respectively performed at 200° C. for 24 hours(Example 1a) and for 72 hours (Example 1b).

After finishing the reaction, the remaining reactant was cooled andwashed sequentially using acetone and methanol.

After washing, the product was dried using a vacuum drier.

After finishing the washing and drying, the reaction product thusobtained was analyzed with an X-ray diffraction spectroscopy and ascanning electron microscope. The reaction product was confirmed to be alithium iron phosphate nanopowder having a particle size of 100 nm±30 nmof pure olivine crystal structure (See FIGS. 1 and 2).

In addition, the particle size distribution of the lithium ironphosphate nanopowder (Example 1b) was measured and is illustrated as agraph in FIG. 3. As shown in the graph, the particle size distributionis uniform.

Example 1c

Each of the lithium iron phosphate nanopowders prepared in Examples 1aand 1b was heat treated at 700° C. for 2 hours to obtain lithium ironphosphate nanopowder including a carbon coating layer formed on thesurface of the nanopowder particle (See FIG. 4).

Example 2

2.5179 g of lithium hydroxide hydrate (LiOH.H₂O), 14.6964 g of ferriccitrate hydrate (FeC₆H_(S)O₇.nH₂O) and 5.88 g of phosphoric acid (H₃PO₄)were added in 300 ml of 1, 4-butandiol and sufficiently stirred toprepare a mixture solution.

The sufficiently stirred mixture solution was put into a 500 ml reactorand a reaction was performed at 215° C. for 72 hours.

After finishing the reaction, the remaining reactant was cooled andwashed sequentially using acetone and methanol. After washing, theproduct was dried using a vacuum drier. After finishing the washing anddrying, the reaction product thus obtained was analyzed with an X-raydiffraction spectroscopy and a scanning electron microscope. Thereaction product was confirmed to be a lithium iron phosphate nanopowderhaving a particle size of about 100 nm of a pure olivine crystalstructure (See FIGS. 5 and 6).

Example 3

6.1212 g of lithium acetate dihydrate (LiCOOLi.2H₂O), 14.6994 g offerric citrate hydrate and 5.88 g of phosphoric acid were added in 300ml of 1,4-butandiol and sufficiently stirred to prepare a mixturesolution.

The sufficiently stirred mixture solution was put into a 500 ml reactorand a reaction was performed at 215° C. for 72 hours.

After finishing the reaction, the remaining reactant was cooled andwashed sequentially using acetone and methanol.

After washing, the product was dried using a vacuum drier.

After finishing the washing and drying, the reaction product thusobtained was analyzed with an X-ray diffraction spectroscopy and ascanning electron microscope. The reaction product was confirmed to be alithium iron phosphate nanopowder having a particle size of about 50 nmof pure olivine crystal structure (See FIGS. 7 and 8).

Comparative Example 1

0.42 g of lithium hydroxide hydrate, 2.45 g of ferric citrate hydrate(FeC₆H_(S)O₇.nH₂O) and 0.98 g of phosphoric acid were added in 50 ml ofethylene glycol and sufficiently stirred to prepare a mixture solution.

The sufficiently stirred mixture solution was put into a 100 mlhydrothermal reactor of high temperature/high pressure and a reactionwas performed at 210° C. for 18 hours.

After finishing the reaction, the remaining reactant was cooled andwashed consecutively using acetone and methanol.

After washing, the product was dried using a vacuum drier.

After finishing the washing and drying, the reaction product thusobtained was analyzed with an X-ray diffraction spectroscopy and ascanning electron microscope. The reaction product was confirmed to be alithium iron phosphate nanopowder having a particle size of about 200 nmto about 1,000 nm and having particle size distribution of low degree ofuniformity (See FIGS. 9 and 10).

As shown through the examples and the comparative example, the lithiumiron phosphate nanopowder prepared by the method of the presentinvention has small and uniform particle size and good particle sizedistribution properties.

While the present invention has been shown and described in connectionwith the exemplary embodiments, it will be apparent to those skilled inthe art that modifications and variations can be made without departingfrom the spirit and scope of the invention as defined by the appendedclaims.

1. A method for preparing a lithium iron phosphate nanopowder,comprising the steps of: (a) preparing a mixture solution by adding alithium precursor, an iron precursor and a phosphorus precursor in areaction solvent; and (b) putting the mixture solution into a reactorand heating to prepare the lithium iron phosphate nanopowder underpressure conditions of 1 to 10 bar.
 2. The method for preparing alithium iron phosphate nanopowder of claim 1, further comprising thestep of (c) heat treating the lithium iron phosphate nanopowder thusprepared to form a coating layer at a portion or a whole of a surface ofan individual particle of the nanopowder.
 3. The method for preparing alithium iron phosphate nanopowder of claim 1, wherein the lithium ironphosphate nanopowder prepared in Step (b) is sequentially conducted awashing step and a drying step.
 4. The method for preparing a lithiumiron phosphate nanopowder of claim 1, wherein the reaction solvent is atleast one selected from the group consisting of 1,2-butandiol,1,3-butandiol, 1,4-butandiol, 2,3-butandiol, and an isomer thereof. 5.The method for preparing a lithium iron phosphate nanopowder of claim 1,wherein the step (b) is performed at a temperature less than or equal toa boiling point of the reaction solvent.
 6. The method for preparing alithium iron phosphate nanopowder of claim 1, wherein Step (b) isperformed at a temperature range of 150 to 235° C.
 7. The method forpreparing a lithium iron phosphate nanopowder of claim 1, wherein Step(b) is performed for 1 to 72 hours.
 8. The method for preparing alithium iron phosphate nanopowder of claim 1, wherein the lithiumprecursor is at least one selected from the group consisting of lithiumacetate dihydrate (CH₃COOLi.2H₂O), lithium hydroxide monohydrate(LiOH.H₂O), lithium hydroxide (LiOH), lithium carbonate (Li₂CO₃),lithium phosphate (Li₃PO₄), lithium phosphate dodecahydrate(Li₃PO₄.12H₂O) and lithium oxalate (Li₂C₂O₄), and a mixture thereof. 9.The method for preparing a lithium iron phosphate nanopowder of claim 1,wherein the iron precursor is at least one selected from the groupconsisting of iron citrate (FeC₆H_(S)O₂), iron citrate hydrate(FeC₆H_(S)O₇.nH₂O), ferrous sulfate heptahydrate (FeSO₄.7H₂O), iron(II)oxalate dihydrate (FeC₂O₄.2H₂O), iron acetyl acetonate (Fe(C₅H₂O₂)₃),iron phosphate dihydrate (FePO₄.2H₂O) and ferric hydroxide (FeO(OH)),and a mixture thereof.
 10. The method for preparing a lithium ironphosphate nanopowder of claim 1, wherein the phosphorus precursor is atleast one selected from the group consisting of tri-ammonium phosphatetrihydrate ((NH₄)₃PO₄.3H₂O), ammonium phosphate ((NH₄)₂HPO₄), ammoniumdihydrogen phosphate (NH₄H₂PO₄) and phosphoric acid (H₃PO₄), and amixture thereof.
 11. The method for preparing a lithium iron phosphatenanopowder of claim 2, wherein the heat treating is performed by heatingto a temperature range of 400 to 900° C.
 12. The method for preparing alithium iron phosphate nanopowder of claim 3, wherein the washing stepis performed by sequentially using acetone and methanol.
 13. The methodfor preparing a lithium iron phosphate nanopowder of claim 3, whereinthe drying is performed at 20 to 160° C. for 2 to 40 hours.
 14. Alithium iron phosphate nanopowder comprising the lithium iron phosphatenanopowder prepared according to claim 1 and having an olivine crystalstructure.
 15. The lithium iron phosphate nanopowder of claim 14,wherein a particle diameter of the lithium iron phosphate nanopowder isfrom 30 to 300 nm.
 16. The lithium iron phosphate nanopowder of claim14, wherein a particle size distribution of the lithium iron phosphatenanopowder is less than or equal to 50% of an average value of theparticle diameter.
 17. The lithium iron phosphate nanopowder of claim14, further comprising a carbon coating layer or a glassy lithiumcompound coating layer on a surface of a particle of the lithium ironphosphate nanopowder.
 18. The lithium iron phosphate nanopowder of claim17, wherein a thickness of the carbon coating layer is less than orequal to 10 nm.
 19. A cathode active material comprising the lithiumiron phosphate nanopowder according to claim
 14. 20. The cathode activematerial of claim 19, further comprising a conductive agent, a binderand a filler.
 21. A cathode for a lithium secondary battery comprisingthe cathode active material of claim
 19. 22. A lithium secondary batterycomprising the cathode of claim 21, an anode, a separator and anon-aqueous electrolyte comprising a lithium salt.
 23. A cathode activematerial comprising the lithium iron phosphate nanopowder according toclaim
 17. 24. A cathode for a lithium secondary battery comprising thecathode active material of claim 23.